In U.S. Pat. No. 4,526,230 to Bozidar Kojicic, there is disclosed a double walled screen filter that includes a pair of substantially spaced concentric screens defining an annular filter space. The screens are connected with a lower perforated plate closing the lower end of the filtering space. The annular space is filled with filtrating materials, such as gravel or synthetic balls, as a filter pack and an upper joint acts as a cover cap of the annular filtering space to close the pack. When installed in a well bore opposite a fluid bearing formation, the lower perforated plate allows for passage of fine particles entering the filter into a sedimentation tube, preventing clogging of the pack and enabling proper functioning of the double walled screen filter. The sedimentation tube is connected below the lower perforated plate and particulate materials enter the filter with formation fluid through the outer screen.
The filter disclosed in the '230 patent provides numerous advantages over prior art filters of which I am aware, these advantages being set forth in the aforesaid patent, the disclosure of which is hereby incorporated by reference herein in its entirety. However, numerous disadvantages also occur with this prior art screen filter.
For example, an important feature of the Kojicic filter is the overall reduction of screen length and diameter due to the possibility of obtaining a high total open area and optimum hydraulic characteristics of the screen filter by virtue of having the openings in the inner and outer screen extend coextensively the entire length of the screen. The filtrating material which is loosely packed gravel or polymer balls fills the entire annular filtrating space, rising to a common elevational position as the upper openings formed in both the inner and outer screens. During production of formation fluid, however, compaction of the filtrating material occurs, resulting in a reduction of height of the pack, and exposing upper openings formed in both the inner and outer screen to a bare filtrating space. Thus, formation fluid entering the filter through the upper openings flows from the outer through the inner screen without being filtered by the pack. This unfiltered fluid containing large sand particles and other particulate material has a tendency to damage pumping and other production equipment when produced to the surface.
Another problem associated with the prior art filter is the tendency of upper portions of the filter pack to clog with sand filtered from the formation fluid. This upper portion is located remote from the lower perforated plate conducting sand from intermediate and lower filter pack portions into the sedimentation tube; however, depending upon hydraulic conditions, sand trapped within the upper portion of the pack cannot always flow through the pack into the sedimentation tube. Hence, clogging occurs.
Since the sand filter is of smaller diameter than the well bore into which it is placed, when such a well is placed on production, sand is eroded from the formation and is carried into the well by the high pressure flow of formation fluid produced concurrently therewith. This entrained sand must initially surround the filter to fill the annular space between the filter and well bore. Normally, the sand stabilizes around the filter. As the term is used herein, sand stabilization basically means settling of large sand grains and other particulate materials around the sand control filter wherein these large sand grains have sufficient open areas to permit flow of formation fluid into the filter. However, when the filter is initially placed within the bottom of the well bore with the well initially put on production, the entrained sand entering the space around the filter tends to severely erode the outer screen due to its abrasive characteristics, often rendering the screen filter useless and requiring immediate replacement.
Replacement of the sand control filter can also be a problem due to the presence of a tightly packed stabilized sand layer produced when the well is initially placed on production and entirely surrounding the filter. In other words, this tightly packed formation sand results in a high coefficient of friction between it and the outer screen of the sand filter requiring the use of heavy lift equipment when removal of the sand filter becomes necessary.
It is accordingly an object of the present invention to provide a sand control filter capable of consistently producing filtered formation fluid in spite of compaction of filtrating materials within the filter.
Another object of the invention is to provide a sand control filter in which clogging of the filtrating pack, particularly upper portions of the pack, does not occur.
Yet another object is to provide a method of installing a sand control filter which prevents abrasion and possible destruction of the outer screen from occurring during the sand stabilization process.
Yet a further object is to provide a method of installing a sand control filter both quickly and easily.
Still another object is to provide a method resulting in fast and easy retrieval of the filter when necessary without requiring heavy lifting equipment and a large labor force.
Yet a further object is to provide a filter which is rugged in design and capable of reliable operation in hostile environments.